1. Field of the Invention
The present invention relates to method and apparatus for splicing together metal webs and, in particular, to such method and apparatus in which the ends of flexible metal webs are spliced together.
2. Description of the Related Art
A conventional metal webs splicing method includes a lap splicing method in which metal webs are in part lapped on each other before they are spliced together and a butt splicing method in which the metal webs are butted against each other before they are spliced together. In FIG. 6(a), there is shown a perspective view of a conventional lap splicing method. As shown in FIG. 6(a), the ends of two metal webs 70 and 72 are lapped on each other and their engagement surfaces 73 are bonded together by a double-side adhesive tape or are fused together by use of ultrasonic waves (FIGS. 4 and 6 of Publication of Laid-Open Japanese Patent Application (Tokkai), No. 59-24526).
Also, in FIG. 6(b), there is shown a perspective view of a conventional metal webs butt splicing method. As shown in FIG. 6(b), the ends of the metal webs 70, 72 are butted against each other and their butt splice portions 75 are spliced together by use of a single-side adhesive tapes 76, 76 (FIG. 2 of Publication of Laid-Open Japanese Patent Application (Tokkai), No. 59-24526).
However, in the conventional metal webs splicing method as shown in FIG. 6(a), there exists a difference in level between the metal webs, that is, between the spliced portions thereof, and, therefore, the spliced metal webs cannot be brought into smooth contact with pass rollers and the like. Also, in a coating step of a photo-sensitive layer in manufacturing a lithographic (planographic) printing plate, when the spliced portions of the metal webs are passed through a coating device, the coating device must be shunted or moved aside in order to prevent against damage. Further, even when the coating device need not be moved aside, the engagement or spliced surfaces 73 have a harmful effect on the state of the coated layer, resulting in a poor quality.
In addition to the above-mentioned disadvantage, in many cases, the spliced surfaces 73 cannot be bonded entirely, with the result that the spliced surface 73 may be turned up while the spliced metal webs are being delivered. Moreover, if there is a process in which the surface of the metal web is treated by use of a treating solution, when the metal web is passing through the treating solution, alien substances such as the treating solution and the like may enter clearances in the spliced surface 73, which gives rise to the generation of a poor quality in following steps.
On the other hand, in the conventional butt splicing method as shown in FIG. 6(b), because the metal webs are spliced together by use of the single-side adhesive tapes 76, 76, the spliced portion produces a level difference in thickness due to the tapes 76, 76. This level difference results in the same disadvantage as in the lap splicing method shown in FIG. 6(a). In such butt splicing method, especially, while the spliced metal webs are being delivered in processes by the pass rollers and the like, the portions of the tapes 76, 76 existing in the butted portions of the metal webs can be shaved off by the edges of the metal webs butted portions to decrease the strength of the tapes 76, 76, so that the adhesive tapes 76, 76, that is, the spliced metal webs may be disadvantageously easy to break. Further, such spliced metal webs are subject to bending stresses while passing through the pass rollers, the spliced portions of the metal webs are easy to break due to such bending stresses. Such breakage may occur outstandingly especially in case of a thin metal web having a thickness of 0.1 mm to 0.2 mm or so and when the two metal webs to be spliced differ greatly in the thicknesses thereof.
In order to solve the above-mentioned problems, there have been disclosed a splicing method in which metal webs are spliced together by welding, while the weldedly spliced portion thereof is rolled to thereby eliminate a level difference between the metal webs, and a splicing method in which an operation to prevent concentration of stresses is performed on a heat affected zone that is produced in the spliced portion due to welding.
In the above-mentioned methods, in general, automatic arc welding is applied and, as shown in FIG. 7, in such arc welding, in most cases, arc is generated while an electrode 86 is maintained in non-contact with metal webs 10, 22. In other words, a mixture gas of He, Ar and the like is flowed into a welding torch 84 and, while the mixture gas is blown down as a shield gas from an opening 85A of a torch cup 85, arc is generated by an arc voltage between the electrode 86 and a back bar 87. Next, the welding torch 84 is moved along the portions to be spliced of the metal webs 10, 22 (in a direction of an arrow A in FIG. 7) to weld and splice the metal webs 10, 22 together. In this case, since the arc is generated while the electrode 86 is maintained in non-contact with the metal webs 10, 22, the leading end of the electrode 86, as shown in FIG. 7, is made in the shape of a sharp edge in order to facilitate the generation of the arc.
However, if the leading end of the electrode 86 is constructed in the form of a sharp edge, as shown in FIG. 7, when the electrode 86 moving in the direction of the arrow A arrives at the metal web, the length of the arc is suddenly decreased from L to L' so that the arc becomes unstable. For this reason, the metal webs cannot be fused sufficiently and thus sufficient both sides welding, that is, welding of not only the front surface but also the back surface of the metal webs cannot be achieved. In other words, at the beginning of the welding, poor fusion may be produced in the weldedly spliced portion and, on the other hand, just after the start of the welding, excessive arc heat may be applied to the metal web, with the result that the metal web may be melted away at the heated portion thereof to produce a perforation there, which is a problem to be solved.
Also, at the end of the welding, the arc length is suddenly increased from L' to L to thereby make the arc unstable, resulting in poor fusion as well as production of perforations.
Further, since there is a time lag of 0.1 sec. to several sec. from the generation of the arc to the time when the arc is changed to a stable arc, if the metal webs 10, 22 are welded and spliced together immediately after the generation of the arc, then the welded splicing of the metal webs is achieved with an arc current which is greater or smaller than a preset value. That is, in case of a thin metal web, just after the generation of the arc, the arc is generated with an arc current greater than the preset value, so that the welding starting portion of the metal web may be melted away. On the other hand, in case of a thick metal web, since the arc is generated with an arc current smaller than the preset value, there can be produced poor fusion or an unfused portion in the welding starting portion of the metal web.
In addition, just after the generation of the arc, the temperature of the electrode 86 is not stable the receiving and transmission of electrons are likely to be unstable, with the result that the arc becomes unstable. Especially, in welding and splicing metal webs together which are formed as thin metal plates, such unstable arc may give rise to uneven beads in the welding starting portion so that poor fusion, perforation, poor appearance (lack of luster) or the like may occur.
The above-mentioned melting-away, poor fusion, unfusion, perforation and the like not only have greatly ill effects on the strength of the weldedly spliced portion, but also, if the following steps include a surface treatment step and/or a coating step, give rise to dirty rollers due to penetration of medical solutions or coating liquid into the back sides of the rollers during the steps and give rise to damaged rollers by the spliced end portions.